(1) Field of the Invention
The present invention relates to intrinsically electrically conductive polymers and polymer blends and to methods for producing intrinsically electrically conductive polymers and polymer blends, and more particularly to such intrinsically conductive polymers and polymer blends having enhanced electrical conductivity, and to methods for producing such polymers and polymer blends having enhanced electrical conductivity.
(2) Description of the Related Art
Electrically conductive polymers are potentially extremely useful because of the combined properties of polymer processiblity coupled with electrical conductivity. It has been known for a long time that polymers can be made conductive by adding conducting additives like carbon black and metal particles but such additives are often highly colored, or opaque, and can also decrease the strength of the polymer.
More recently, it has been found that some polymers have appreciable electrical conductivity as an intrinsic property and can act as electrical conductors without the addition of conducting additives. Such polymers have been termed intrinsically conductive polymers (ICP's). Although many polymers have now been identified which act as ICP's, such polymers as polyaniline, polypyrrole, polythiophene, polyacetylene, and derivatives thereof, are characteristic. As used herein, the terms intrinsically conductive polymer, and ICP, will be understood to mean an organic polymer that contains polyconjugated bond systems and which can be doped with electron donor dopants or electron acceptor dopants to form a charge transfer complex that has an electrical conductivity of at least about 10.sup.-8 S/cm. Such doped ICP's will be referred to herein as the salts of intrinsically conductive polymers, or ICP salts. It will be understood that whenever an electrically conductive ICP is referred to herein, it is meant that the material is an ICP salt. Comprehensive reviews of ICP technology can be found in Synthetic Metals, vols. 17-19, 1987; vols. 28-30, 1989; and vols. 40-42, 1991.
Polyaniline has been of special interest due to its low cost, high stability under ambient conditions and capacity for being easily rendered conductive. Polyaniline occurs in at least four oxidation states: leuco-emeraldine, emeraldine salt, nigraniline and pernigraniline. Of these, the emeraldine salt form is the only electrically conductive state. In polyaniline, the presence or absence of a protonic acid dopant (counterion) can change the state of the polymer, respectively, from emeraldine salt to emeraldine base. Thus, the presence or absence of such a dopant can reversibly render the polymer conductive, or non-conductive. The use of protonic acids as dopants for ICP's such as polyaniline is well known and simple protonic acids such as HCl and H.sub.2 SO.sub.4, or with functionalized organic acids such as para-toluenesulfonic acid (pTSA) or dodecylbenzenesulfonic acid (DBSA) results in the formation of conductive polyaniline.
Kahol et al., in Synth. Met., 84:691-694, 1997, reported that it is well known that the electrical conductivity of protonated polyaniline is enhanced due to the presence of moisture. Although several theories have been proposed to explain this effect, the mechanism is not known. See, e.g., Chiang, J.-C. and A. G. MacDiarmid, Synth. Met., 13:193, 1986; Angelopoulos, M. et al., Synth. Met., 21:21, 1987; Nechtschein, M., and C. Santier, J. Phys., 47:935, 1986; Focke, W. W., and G. E. Wnek, J. Electroanal. Chem. 256:343, 1988; Javadi, H. H. S. et al., Synth. Met., 26:1, 1988; Timofeeva, O. N. et al., Synth. Met., 40:111, 1991 and Shacklette, L. W., Synth. Met., 65:123, 1994.
Although electrical conductivity is often a key property of the final product of an ICP, one drawback has been that ICP's in their conductive forms are often difficult to process. Earlier, for example, it was felt that polyaniline, in doped, or salt, form was insoluble in all organic solvents and in neutral form, was soluble only in highly polar solvents, such as N-methyl pyrrolidone. More recently it has been found that certain methods of synthesis, and the use of certain functionalized organic acid dopants, render electrically conductive polyaniline salt more soluble in non-polar organic solvents. See, e.g., U.S. Pat. No. 5,567,356, (use of hydrophobic counterions in emulsion polymerization with polar organic liquids) and WO 92/22911 and U.S. Pat. Nos. 5,324,453 and 5,232,631, (use of counterions having surfactant properties in emulsion polymerization with non-polar organic liquids). One potential drawback of polyaniline salts with high organic solubility can be the difficulty of providing sufficient moisture to the polyaniline to retain the desirably high electrical conductivity in highly organic systems.
In addition to solubility limitations of ICP's, melt processing of ICP's has also been difficult due to their tendency to destruct thermally prior to melting. Polymer blending has been tried, but conductivity is not always preserved, or may decrease to unacceptably low levels. Various additives such as surfactants, compatibilizers and/or plasticizers have been added to ICP's and blends to improve processibility and other mechanical properties, but many of these additives reduce the electrical conductivity. Consequently, recent experimentation has focused on ways to improve various processibility and mechanical properties of ICP's, while maintaining or improving the electrical conductivity.
For example, it is well known to use plasticizers in thermoset and thermoplastic polymers to improve the workability, flexibility, distensibility and/or impact resistance of the polymer during forming or in the final product. Plasticizers have also been added to ICP's to improve processibility. For example, Laska, J. et al., J. Appl. Poly. Sci., 61:1339-1343, 1996, investigated the rheological behavior of polyaniline plasticized with diisooctyl phosphate. Chen, S.-A., and Lee, H.-T., Macromolecules, 26:3254-3261, 1993, reported the structure and doping behavior of polyaniline plasticized with 1-methyl-2-pyrrolidone (NMP). Later, the same group, Lee, H.-T. et al., Macromolecules, 28:7645-7652, 1995, reported the effect of NMP doping of polyaniline on the conductivity relaxation of films cast from the material.
U.S. Pat. No. 5,171,478, to Han, disclosed a method for increasing the molecular weight of polyaniline by a controlled heating process. It was reported that shorter heating times could be employed by including a plasticizer with the polyaniline. It was believed that the plasticizer increased intrachain mobility of the polyaniline and facilitated chain coupling reactions resulting in polyanilines of higher molecular weight. An extensive list of possible plasticizing agents was provided, including several organic acids well known as dopants for polyaniline, but the effect of such plasticizers on electrical conductivity was not reported.
European Patent Application Publ. No. 0582 919 A2 reported the use of metals complexed with protonic acids as compositions useful for neutralizing, plasticizing, lowering the percolation threshold of, and/or stabilizing polyanilines, or derivatives thereof, which had been doped with a protonic acid. The preferred composition, a zinc oxide--dodecylbenzenesulfonic acid (ZnO-DBSA) complex, was added to polyaniline doped with DBSA. Improved melt-processing properties and increased electrical conductivity were reported for such materials and for blends with thermoplastics. However, large amounts of organic acids were used to form the subject compositions, since high levels of acids were used in both the acid-doped polyaniline and in the metal-acid complex.
In U.S. Pat. No. 5,217,649 and EP 497 514 A1, Kulkarni et al., reported the formation of conductive polymer blends containing an ICP and a thermoplastic polymer along with an acidic surfactant and from about 1% wt/wt to about 40% wt/wt of a highly polar, ester-free plasticizer such as a sulfonamide. In blends with polyvinylchloride and a sulfonamide plasticizer, polyaniline added from about 5% wt/wt to about 50% wt/wt of the blend resulted in a concommitant increase in electrical conductivity of the blend. A maximum conductivity of about 10 S/cm was reached at a polyaniline level of about 33% and a sulfonamide level of about 17%. The reference did not mention the moisture level of the compositions or the organic solubility of the polyaniline.
Later, in U.S. Pat. No. 5,290,483, the same group of investigators disclosed a process for producing a conductive polymer article. In one example, a sulfonamide plasticizer was used as an additive to polyaniline along with carbon black, chlorinated polyethylene, an organic surfactant and a stabilizer. As before, however, the moisture level of the compositions was not mentioned.
More recently, Kulkarni et al., in U.S. Pat. No. 5,595,689, taught the production of conductive ICP blends that contained non-polymeric polar additives such as sulfonamides in general and N-butylbenzene sulfonamide in particular. All ICP blends were produced with the highly polar polyaniline, VERSICON.RTM., that is known to have low solubility in non-polar organic liquids. No examples indicate the processibility of the products in organic solvents and no mention was made of methods to retain moisture in the processed materials.
The addition of surfactants, plasticizers and other processing aids to ICP blends with thermoplastic and thermosetting resins was mentioned in U.S. Pat. No. 5,494,609, but the purpose of such additives was to improve the dispersion and film forming properties of the polymer dispersions rather than to modify the conductivity. Highly polar plasticizers such as sulfonamides, phosphate and benzoate esters were reported to be preferred for use with ICP blends with thermoplastics, but with the purpose of producing optimum film forming quality. However, Pron, A. et al., J. Appl. Polym. Sci., 63:971-977, 1997, disclosed that a mixture of phthalic and phosphoric acid esters improved the flexibility and also the conductivity of films cast from protonated polyaniline blends with cellulose acetate in m-cresol solutions. The addition of plasticizers significantly lowered the percolation threshold of the polyaniline/cellulose acetate blends. The polyaniline used in the study had been synthesized by standard aqueous oxidative polymerization and the emeraldine base was subsequently doped with such protonating agents as, camphor sulfonic acid, phenyl-phosphonic acid, dibutyl phosphate, dioctyl phosphate and diphenyl phosphate. However, the resulting polyaniline salt demonstrated a maximum solubility in m-cresol of less than 0.5% and solubility in such non-polar solvents as xylene would probably have been significantly lower.
Laska, J. et al., Materials Science Forum, 122:177-184, 1993, reported that the solubilization and plastification of polyaniline in the protonated state was improved by the addition of di-alkyl phosphate esters. Esters such as diisoctyl hydrogen phosphate and diisobutyl hydrogen phosphate served to protonate the polyaniline as well as to serve as plasticizers. Although the plasticized compounds were termed soluble in common solvents, the highest solubility in toluene or decalin was apparently under 10% wt/wt. Polyanilines complexed with such aliphatic esters, however, did not include any acid salts.
Later, the same group, Laska, J., et al., J. Polym. Sci.: Part A: Polym. Chem., 33:1437-1445, 1995, expanded the types of phosphoric acid diesters studied. Again it was reported that the diesters protonated the polyaniline base as well as served to plasticize the complex. But, again none of the aliphatic phosphate diester-plasticized compounds were the acid salts of ICP's. It was stated that the phosphoric acid diesters act as efficient polyaniline solubilizing agents and that diester protonated polyanilines readily dissolved in solvents such as toluene and xylene, but no level of solubility was given for these materials.
Ikkala et al., U.S. Pat. No. 5,520,852, found that non-functional acids, such as hydrochloric acid and H.sub.2 SO.sub.4, can be used as counter-ions for polyaniline complexes and polymer blends if certain organic cyclic compounds are also present. They disclosed that such organic cyclic compounds work as solvent-plasticizers in doped polyanilines where the doping counter-ion is not functionalized, and as compatibilizers where polyaniline is blended with another polymer. The organic cyclic compounds are disclosed to be sulfonamides or molecular recognition compounds capable of bonding with the six-membered rings of the polyaniline salt complexes. Since the purpose of the organic cyclic compounds was to facilitate the use of non-functional, protonic acids, such compounds were not added to polyanilines having functional organic acid dopants. No mention was made of the desirability or effect of moisture level in the compositions.
In U.S. Pat. No. 5,585,038, to Kirmanen et al., a method was disclosed for preparing an electrically conductive doped polyaniline compound which contained the polyaniline, a doping acid such as DBSA, a metal compound such as zinc oxide, a base such as calcium carbonate and a plasticizing agent such as water, C.sub.1 -C.sub.3 alcohols and mixtures thereof. Such compound was reported to have low acidity, be easily processible at elevated temperatures and to be electrically conductive. Despite the addition of water, or low boiling alcohols, at the initial stage of forming the mixture, the final composition as manufactured would not contain a significant amount of such volatile plasticizing component because high temperatures employed during a subsequent melt-mixing step cause them to be driven off. The moisture level in the final composition was not disclosed to be important to the properties of the composition.
Although it is known that the electrical conductivity of ICP's, and especially polyaniline, can be enhanced by increasing the moisture level in the composition, no method is available to obtain enhanced levels of conductivity in ICP compositions having high organic solubility and to retain the enhanced level of conductivity under conditions that leach out water soluble components. Methods to maintain such enhanced conductivity, while at the same time enhancing the processibility and mechanical properties of such organically soluble ICP's, would be extremely useful for the production of ICP's and polymer blends containing ICP's having high organic solubility, improved mechanical properties and enhanced electrical conductivity.